The present invention relates to a process for the recovery and purification of an olefinically unsaturated nitrite, such as acrylonitrile, from a reactor effluent stream.
Processes for the production of olefinically unsaturated nitrites by the catalytic reaction of ammonia and an olefin are well known. For example, acrylonitrile and methacrylonitrile may be produced by the vapor phase catalytic oxidation of propylene and isobutylene, respectively, in the presence of ammonia.
In commercial processes for preparation of acrylonitrile from propylene, ammonia, and oxygen, the reactor effluent contains, in addition to the desired acrylonitrile product, considerable amounts of by-product hydrogen cyanide, acetonitrile, and other impurities such as succinonitrile and other nitrites. The exact composition of the effluent and the by-products and impurities it contains may vary considerably depending on the ammoxidation reaction conditions and catalyst. Reactor effluents from processes for producing other olefinically unsaturated nitrites similarly contain various byproducts or impurities.
Processes for treating reactor effluents of the type described to separate and recover acrylonitrile product from by-products or impurities are known. For example, see U.S. Pat. Nos. 3,399,120, 3,433,822, 3,936,360, 4,059,492, 4,166,008, and 4,404,064, which are incorporated herein by reference. Typically, these processes include introducing the reactor effluent into a quench chamber where it is contacted with water (usually containing sulfuric acid to neutralize excess ammonia from the reaction) to cool the effluent and remove some contaminants such as polymers produced in the reactor. Cooled effluent gases from the quench flow to an absorber column where they are contacted with water. The liquid stream from the bottom of the absorber column contains most of the nitrites produced in the reaction and some impurities, and is sent to an extractive distillation column, also referred to as the recovery column. The major portion of the acrylonitrile from the extractive distillation column is obtained in the overhead (distillate) from the recovery column while water and impurities constitute most of the bottom stream from the recovery column. The bottom stream is typically fed to a secondary distillation or stripper column to separate acetonitrile and water in an overhead stream while the secondary column bottoms containing water and various impurities are recycled, for example to the quench column.
U.S. Pat. No. 3,399,120 describes an embodiment in which acrylonitrile is purified utilizing a single recovery and stripper column. However, an enrichment column is required to concentrate and separate the acetonitrile from the acrylonitrile and water mixture.
The use of two or more columns, the recovery column, the stripper column and/or the enrichment column, is effective to achieve the product separation and recovery that is required in commercial operations. However, this system is expensive, due both to the cost of the equipment involved (not only the two or more columns, but also the associated pumps, piping, heat exchangers, etc.) and the operating costs such as energy usage by the two columns. A need exists for improved processes that can achieve the desired recovery at a lower cost.